On Friday, January 24, 2014 9:55:53 AM UTC-5, Joe Gwinn wrote:   
   > In article , Phil Hobbs   
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   >  wrote:   
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   > > On 01/23/2014 09:50 AM, Joe Gwinn wrote:   
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   > > > In article , Phil Hobbs   
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   > > >  wrote:   
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   > > >   
   >   
   > > >> Hi, all,   
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   > > >>   
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   > > >> I have a gig to design a microplate reader for a new bioassay system.   
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   > > >> To match the reagent systems, it needs to work over a range of   
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   > > >> wavelengths in the 340-500 nm region, none of which is particularly well   
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   > > >> matched to mercury emission lines.   
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   > > >>   
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   > > >> So, I'm casting about for a light source.  It really doesn't need much   
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   > > >> power, maybe a few milliwatts per square cm in a 5-nm passband.   So   
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   > > >> 5-10 W output would be fine for an arc lamp, much less for a LED.   
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   > > >>   
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   > > >> For particular purposes, I can get LEDs in almost any wavelength I need.   
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   > > >>    However, it would be very useful to have a broadband source.   
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   > > >>   
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   > > >> Most white LEDs appear to cut off very sharply below about 420 nm, which   
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   > > >> is pretty understandable given that that's the short wavelength tail of   
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   > > >> the blue LED chip.   
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   > > >>   
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   > > >> High pressure xenon lamps have nearly flat spectra in that region, which   
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   > > >> would be terrific if I could find one rated at less than a kilowatt.   
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   > > >>   
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   > > >> Any lamp- or LED-selection wisdom?   
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   > > > How about a pulsed xenon flashlamp, ie, a stroboscope?  These are   
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   > > > easily built.  (I built one to stop motion in a coil winder, so one   
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   > > > could diagnose winding problems.  The flash is triggered from a axle   
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   > > > position sensor, so the image stands still regardless of rotation   
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   > > > speed.)  Pulsed at low power (relative to the capacity of the flashlamp   
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   > > > in question), the life can be quite long.  The pulsed output fits into   
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   > > > lock-in amplifier schemes nicely.   
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   > > > Joe Gwinn   
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   > > >   
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   > > Thanks, Joe.   
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   > > That's the approach that a lot of existing microplate readers use.  The   
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   > > problem is the pulse-to-pulse variation, which hurts the measurement   
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   > > repeatability.  Something nice and stable like a LED would be my first   
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   > > choice.  I may wind up with a white LED with a violet and a UV one for   
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   > > fill-in, but that's a bit on the messy side and tends to waste light.   
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   > >   
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   > > The sample will be in rapid motion during the measurement, so an arc   
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   > > lamp is a possibility.   
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   > > There's some specification creep happening at the moment.  Originally it   
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   > > was a single-wavelength system, where a filtered LED would have been   
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   > > just the ticket, but now it looks more like a fibre-coupled spectrometer.   
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   > If pulse-to-pulse variation is the problem, I'd be tempted to have a   
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   > two-beam setup, using one beam as the reference and the other for   
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   > measurements, and taking their ratio instant by instant.  This should   
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   > largely cancel variation over pulse time, pulse amplitude, and pulse   
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   > shape.   
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   > The copious optical power available from a flashlamp makes up for a   
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   > host of other sins.   
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   > Joe Gwinn   
      
   unless a PI for a divorce lawyer catches you in his camera flash in a moment   
   in flagrante indelicato. Flashback to Philip Marlowe, PI (Raymond Chandler)   
      
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